Hypertext Transfer Protocol (HTTP/1.1): Range Requests

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document defines range requests and the rules for constructing and combining responses to those requests.¶

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.¶

Hypertext Transfer Protocol (HTTP) clients often encounter interrupted data transfers as a result of canceled requests or dropped connections. When a client has stored a partial representation, it is desirable to request the remainder of that representation in a subsequent request rather than transfer the entire representation. Likewise, devices with limited local storage might benefit from being able to request only a subset of a larger representation, such as a single page of a very large document, or the dimensions of an embedded image.¶

This document defines HTTP/1.1 range requests, partial responses, and the multipart/byteranges media type, obsoleting those parts previously defined in [RFC2616]. Range requests are an OPTIONAL feature of HTTP, designed so that recipients not implementing this feature (or not supporting it for the target resource) can respond as if it is a normal GET request without impacting interoperability. Partial responses are indicated by a distinct status code to not be mistaken for full responses by caches that might not implement the feature.¶

Although the range request mechanism is designed to allow for extensible range types, this specification only defines requests for byte ranges.¶

A representation can be partitioned into subranges according to various structural units, depending on the structure inherent in the representation's media type. This "range unit" is used in the Accept-Ranges (Section 2.3) response header field to advertise support for range requests, the Range (Section 3.1) request header field to delineate the parts of a representation that are requested, and the Content-Range (Section 4.2) payload header field to describe which part of a representation is being transferred.¶

Since representation data is transferred in payloads as a sequence of octets, a byte range is a meaningful substructure for any representation transferable over HTTP (Section 3 of [Part2]). We define the "bytes" range unit for expressing subranges of the data's octet sequence.¶

The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero.¶

A client can limit the number of bytes requested without knowing the size of the selected representation. If the last-byte-pos value is absent, or if the value is greater than or equal to the current length of the representation data, the byte range is interpreted as the remainder of the representation (i.e., the server replaces the value of last-byte-pos with a value that is one less than the current length of the selected representation).¶

In the byte range syntax, first-byte-pos, last-byte-pos, and suffix-length are expressed as decimal number of octets. Since there is no predefined limit to the length of a payload, recipients ought to anticipate potentially large decimal numerals and prevent parsing errors due to integer conversion overflows.¶

The "Range" header field on a GET request modifies the method semantics to request transfer of only one or more subranges of the selected representation data, rather than the entire selected representation data.¶

A server MAY ignore the Range header field. However, origin servers and intermediate caches ought to support byte ranges when possible, since Range supports efficient recovery from partially failed transfers and partial retrieval of large representations. A server MUST ignore a Range header field received with a request method other than GET.¶

An origin server MUST ignore a Range header field that contains a range unit it does not understand. A proxy MAY either discard a Range header field that contains a range unit it does not understand or pass it to the next inbound server when forwarding the request.¶

A server that supports range requests ought to ignore or reject a Range header field that consists of more than two overlapping ranges, or a set of many small ranges that are not listed in ascending order, since both are indications of either a broken client or a deliberate denial of service attack (Section 6.1). A client SHOULD NOT request multiple ranges that are inherently less efficient to process and transfer than a single range that encompasses the same data.¶

A client that is requesting multiple ranges SHOULD list those ranges in ascending order (the order in which they would typically be received in a complete representation) unless there is a specific need to request a later part earlier. For example, a user agent processing a large representation with an internal catalog of parts might need to request later parts first, particularly if the representation consists of pages stored in reverse order and the user agent wishes to transfer one page at a time.¶

The Range header field is evaluated after evaluating the preconditions of [Part4] and only if the result of their evaluation is leading toward a 200 (OK) response. In other words, Range is ignored when a conditional GET would result in a 304 (Not Modified) response.¶

The If-Range header field (Section 3.2) can be used as a precondition to applying the Range header field.¶

If all of the preconditions are true, the server supports the Range header field for the target resource, and the specified range(s) are valid and satisfiable (as defined in Section 2.1), the server SHOULD send a 206 (Partial Content) response with a payload containing one or more partial representations that correspond to the satisfiable ranges requested, as defined in Section 4.¶

If all of the preconditions are true, the server supports the Range header field for the target resource, and the specified range(s) are invalid or unsatisfiable, the server SHOULD send a 416 (Range Not Satisfiable) response.¶

If a client has a partial copy of a representation and wishes to have an up-to-date copy of the entire representation, it could use the Range header field with a conditional GET (using either or both of If-Unmodified-Since and If-Match.) However, if the condition fails because the representation has been modified, the client would then have to make a second request to obtain the entire current representation.¶

The "If-Range" header field allows a client to "short-circuit" the second request. Informally, its meaning is: if the representation is unchanged, send me the part(s) that I am requesting in Range; otherwise, send me the entire representation.¶

Clients MUST NOT use an entity-tag marked as weak in an If-Range field value and MUST NOT use a Last-Modified date in an If-Range field value unless it has no entity-tag for the representation and the Last-Modified date it does have for the representation is strong in the sense defined by Section 2.2.2 of [Part4].¶

A server that evaluates a conditional range request that is applicable to one of its representations MUST evaluate the condition as false if the entity-tag used as a validator is marked as weak or, when an HTTP-date is used as the validator, if the date value is not strong in the sense defined by Section 2.2.2 of [Part4]. (A server can distinguish between a valid HTTP-date and any form of entity-tag by examining the first two characters.)¶

A client MUST NOT generate an If-Range header field in a request that does not contain a Range header field. A server MUST ignore an If-Range header field received in a request that does not contain a Range header field. An origin server MUST ignore an If-Range header field received in a request for a target resource that does not support Range requests.¶

If the validator given in the If-Range header field matches the current validator for the selected representation of the target resource, then the server SHOULD process the Range header field as requested. If the validator does not match, then the server MUST ignore the Range header field.¶

The 206 (Partial Content) status code indicates that the server is successfully fulfilling a range request for the target resource by transferring one or more parts of the selected representation that correspond to the satisfiable ranges found in the requests's Range header field (Section 3.1).¶

If a single part is being transferred, the server generating the 206 response MUST generate a Content-Range header field, describing what range of the selected representation is enclosed, and a payload consisting of the range. For example:¶

If multiple parts are being transferred, the server generating the 206 response MUST generate a "multipart/byteranges" payload, as defined in Appendix A, and a Content-Type header field containing the multipart/byteranges media type and its required boundary parameter. To avoid confusion with single part responses, a server MUST NOT generate a Content-Range header field in the HTTP header block of a multiple part response (this field will be sent in each part instead).¶

Within the header area of each body part in the multipart payload, the server MUST generate a Content-Range header field corresponding to the range being enclosed in that body part. If the selected representation would have had a Content-Type header field in a 200 (OK) response, the server SHOULD generate that same Content-Type field in the header area of each body part. For example:¶

When multiple ranges are requested, a server MAY coalesce any of the ranges that overlap or that are separated by a gap that is smaller than the overhead of sending multiple parts, regardless of the order in which the corresponding byte-range-spec appeared in the received Range header field. Since the typical overhead between parts of a multipart/byteranges payload is around 80 bytes, depending on the selected representation's media type and the chosen boundary parameter length, it can be less efficient to transfer many small disjoint parts than it is to transfer the entire selected representation.¶

A server MUST NOT generate a multipart response to a request for a single range, since a client that does not request multiple parts might not support multipart responses. However, a server MAY generate a multipart/byteranges payload with only a single body part if multiple ranges were requested and only one range was found to be satisfiable or only one range remained after coalescing. A client that cannot process a multipart/byteranges response MUST NOT ask for multiple ranges in a single request.¶

When a multipart response payload is generated, the server SHOULD send the parts in the same order that the corresponding byte-range-spec appeared in the received Range header field, excluding those ranges that were deemed unsatisfiable or that were coalesced into other ranges. A client that receives a multipart response MUST inspect the Content-Range header field present in each body part in order to determine which range is contained in that body part; a client cannot rely on receiving the same ranges that it requested, nor the same order that it requested.¶

If a 206 is generated in response to a request with an If-Range header field, the sender SHOULD NOT generate other representation header fields beyond those required above, because the client is understood to already have a prior response containing those header fields. Otherwise, the sender MUST generate all of the representation header fields that would have been sent in a 200 (OK) response to the same request.¶

The "Content-Range" header field is sent in a single part 206 (Partial Content) response to indicate the partial range of the selected representation enclosed as the message payload, sent in each part of a multipart 206 response to indicate the range enclosed within each body part, and sent in 416 (Range Not Satisfiable) responses to provide information about the selected representation.¶

If a 206 (Partial Content) response contains a Content-Range header field with a range unit (Section 2) that the recipient does not understand, the recipient MUST NOT attempt to recombine it with a stored representation. A proxy that receives such a message SHOULD forward it downstream.¶

For byte ranges, a sender SHOULD indicate the complete length of the representation from which the range has been extracted, unless the complete length is unknown or difficult to determine. An asterisk character ("*") in place of the complete-length indicates that the representation length was unknown when the header field was generated.¶

The following example illustrates when the complete length of the selected representation is known by the sender to be 1234 bytes:¶

Content-Range: bytes 42-1233/1234

and this second example illustrates when the complete length is unknown:¶

The complete-length in a 416 response indicates the current length of the selected representation.¶

The "Content-Range" header field has no meaning for status codes that do not explicitly describe its semantic. For this specification, only the 206 (Partial Content) and 416 (Range Not Satisfiable) status codes describe a meaning for Content-Range.¶

The following are examples of Content-Range values in which the selected representation contains a total of 1234 bytes: ¶

A response might transfer only a subrange of a representation if the connection closed prematurely or if the request used one or more Range specifications. After several such transfers, a client might have received several ranges of the same representation. These ranges can only be safely combined if they all have in common the same strong validator, where "strong validator" is defined to be either an entity-tag that is not marked as weak (Section 2.3 of [Part4]) or, if no entity-tag is provided, a Last-Modified value that is strong in the sense defined by Section 2.2.2 of [Part4].¶

A client that has received multiple partial responses to GET requests on a target resource MAY combine those responses into a larger continuous range if they share the same strong validator.¶

If the most recent response is an incomplete 200 (OK) response, then the header fields of that response are used for any combined response and replace those of the matching stored responses.¶

If the most recent response is a 206 (Partial Content) response and at least one of the matching stored responses is a 200 (OK), then the combined response header fields consist of the most recent 200 response's header fields. If all of the matching stored responses are 206 responses, then the stored response with the most recent header fields is used as the source of header fields for the combined response, except that the client MUST use other header fields provided in the new response, aside from Content-Range, to replace all instances of the corresponding header fields in the stored response.¶

The combined response message body consists of the union of partial content ranges in the new response and each of the selected responses. If the union consists of the entire range of the representation, then the client MUST record the combined response as if it were a complete 200 (OK) response, including a Content-Length header field that reflects the complete length. Otherwise, the client MUST record the set of continuous ranges as one of the following: an incomplete 200 (OK) response if the combined response is a prefix of the representation, a single 206 (Partial Content) response containing a multipart/byteranges body, or multiple 206 (Partial Content) responses, each with one continuous range that is indicated by a Content-Range header field.¶

The 416 (Range Not Satisfiable) status code indicates that none of the ranges in the request's Range header field (Section 3.1) overlap the current extent of the selected resource or that the set of ranges requested has been rejected due to invalid ranges or an excessive request of small or overlapping ranges.¶

For byte ranges, failing to overlap the current extent means that the first-byte-pos of all of the byte-range-spec values were greater than the current length of the selected representation. When this status code is generated in response to a byte range request, the sender SHOULD generate a Content-Range header field specifying the current length of the selected representation (Section 4.2).¶

Note: Because servers are free to ignore Range, many implementations will simply respond with 200 (OK) if the requested ranges are invalid or not satisfiable. That is partly because most clients are prepared to receive a 200 (OK) to complete the task (albeit less efficiently) and partly because clients might not stop making an invalid partial request until they have received a complete representation. Thus, clients cannot depend on receiving a 416 (Range Not Satisfiable) response even when it is most appropriate.¶

This section is meant to inform developers, information providers, and users of known security concerns specific to the HTTP/1.1 range request mechanisms. More general security considerations are addressed in HTTP messaging [Part1] and semantics [Part2].¶

Unconstrained multiple range requests are susceptible to denial of service attacks because the effort required to request many overlapping ranges of the same data is tiny compared to the time, memory, and bandwidth consumed by attempting to serve the requested data in many parts. Servers ought to ignore, coalesce, or reject egregious range requests, such as requests for more than two overlapping ranges or for many small ranges in a single set, particularly when the ranges are requested out of order for no apparent reason. Multipart range requests are not designed to support random access.¶

When a 206 (Partial Content) response message includes the content of multiple ranges, they are transmitted as body parts in a multipart message body ([RFC2046], Section 5.1) with the media type of "multipart/byteranges". The following definition is to be registered with IANA [BCP13].¶

The multipart/byteranges media type includes one or more body parts, each with its own Content-Type and Content-Range fields. The required boundary parameter specifies the boundary string used to separate each body part.¶

Although [RFC2046] permits the boundary string to be quoted, some existing implementations handle a quoted boundary string incorrectly.

A number of clients and servers were coded to an early draft of the byteranges specification that used a media type of multipart/x-byteranges, which is almost (but not quite) compatible with this type.

Despite the name, the "multipart/byteranges" media type is not limited to byte ranges. The following example uses an "exampleunit" range unit:¶